Field of the Invention
The present invention relates to an oscillating circuit using, for example, a quartz crystal resonator, a Surface Acoustic Wave (SAW) resonator, or a ceramic resonator. Moreover, the present invention relates to a semiconductor integrated circuit having such an oscillating circuit.
Demand for faster operating speeds in computers and other electronic equipment increases each year. Electronic equipment controls the timing of operations in circuits in all parts, using a clock signal generated by an internal oscillating circuit. Therefore, in order to increase the operating speed of such electronic equipment, it is necessary to raise the oscillating frequency in oscillating circuits. On the other hand, when the oscillating frequency is raised, power consumption and heat generation tend to increase. Hence, designers seek to minimize the power consumption of oscillating circuits.
Oscillating circuits require high frequency stability. A crystal oscillator having a crystal resonator is the most commonly used type of oscillating circuit. Crystal resonators exhibit a property wherein the thinner the crystal, the higher its oscillation frequency. However, the thinner the crystal, the more difficult is its industrial mass production.
Therefore, rather than use the fundamental mode of a quartz crystal, oscillating circuits that operate on an odd overtone of the fundamental mode are used. This type of oscillating circuit is called an overtone oscillating circuit.
A 3rd overtone oscillating circuit wherein oscillation in an overtone mode is enabled by lowering the feedback resistor of an oscillating circuit is shown in FIG. 5. In this oscillating circuit a crystal resonator 50, an inverter 53, and a feedback resistor 54 are connected in parallel, and capacitors 51, 52 are connected respectively between junctions and ground potential.
According to FIG. 5, if the resistance value of the feedback resistor 54 is set at or below a certain limit, the oscillating circuit will no longer be able to oscillate in the fundamental mode and will oscillate in the overtone mode. The resistance value of that limit is determined by the frequency of the fundamental wave of the crystal resonator 50 used, and the higher the fundamental wave, the smaller the resistance value of the limit at which that fundamental wave is no longer able to oscillate. For example, in the case of a crystal resonator having a fundamental wave frequency of about 10 MHz, the resistance value at which oscillation is no longer possible in the fundamental mode is approximately 30 kΩ.
Therefore, according to the oscillating circuit shown in FIG. 5, for example, to obtain an oscillating frequency of 36 MHz, the fundamental wave of the crystal resonator 50 is set to 12 MHz, the resistance value of the feedback resistor 54 is set to 30 kΩ or less, and oscillation in the 3rd overtone mode is achieved.
On the other hand, FIG. 6 discloses a 3rd overtone oscillating circuit in which the gain of an inverting amplifier is increased and oscillation startup performance is enhanced. In this oscillating circuit, an output of a first inverter 61 is connected to a gate of a second inverter 62, and an output of the second inverter 62 is connected to a gate of a third inverter 63. Moreover, an inverting amplifier is formed by connecting a feedback resistor 64 having a resistance value below a certain limit between an output of the third inverter 63 and a gate of the first inverter 61. This oscillating circuit is similar to that shown in FIG. 5 in that the oscillating circuit is formed by connecting this feedback resistor 64 and crystal resonator 50 in parallel and, connecting capacitors 51, 52 respectively between these junctions and ground potential.
According to FIG. 6, an ordinary AT-cut crystal resonator has a strong predisposition toward oscillating at the frequency of the fundamental wave, and it is difficult to achieve oscillation in the 3rd overtone mode simply by shrinking the resistance value of the feedback resistor as described above. Hence and it is therefore necessary to increase the driving force of the inverting amplifier and to enhance the oscillation startup. However, if an inverter having a large driving force is used in order to enhance oscillation startup, the problem of increased current consumption occurs.
Therefore, in the oscillating circuit shown in FIG. 6, the input signal of inverter 61 is greatly amplified by inverters 61-63 so as to enable oscillation in the 3rd overtone mode, even if the driving force of the individual inverters is small. In addition, due to the waveform shaping effect of the inverter, the gate input waveform of the second inverter 62 and of the third inverter 63 become closer to rectangular waves. These make it possible to diminish the overall level of the oscillating circuit's current consumption.
However, according to the oscillating circuit shown in FIG. 6, the gain of an inverted amplifying circuit including inverters 61-63 is extremely large, so even very slight noise input to the inverter 61 is greatly amplified, making the oscillating circuit highly susceptible to the effects of noise.
Moreover, at high frequencies, the phase rotation of an inverted amplifying circuit including inverters 61-63 grows large, making the amplifying circuit susceptible to the adverse effects of temperature drifts and circuit element variations and resulting in unstable oscillation.
Therefore, taking the aforementioned problems into account, the present invention, of an oscillating circuit using an oscillating element such as a crystal resonator, is intended to diminish the effects of temperature drifts and circuit element variations and to enable stable oscillation at high frequencies while limiting current consumption.